It could be several things:
he could scratched his eye on something.
He could have an infection
He could have too much fat in the diet.

What is he eating, and how much? I would suggest taking in a stool sample to a vet to test for any infections. Call up the vet tell him you need a stool sample done. Basically you want to just bring in the sample not the frog. When a frog is sick the stress of being removed from the case and put in a container and then taken to a vet can kill it, so it should only be done as a last resort.